Example with duplicates
Input
remove_duplicates([1, 1, 2])
Output
2
The unique elements are [1, 2], so the function returns 2.
Full lesson preview
Remove duplicates from a sorted array
Remove duplicates in-place from a sorted array and return the count of unique elements.
Python practice10 minArrays – Fundamentals & PatternsBeginnerLast updated March 25, 2026
Problem statement
Given a sorted list nums, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same. You should not allocate extra space for another list; modify the input list in-place with O(1) extra memory. Return the number of unique elements (an integer).
Task
Implement an in-place algorithm to remove duplicates from a non-decreasing sorted list and return the new length (number of unique elements).
Examples
Input format
A single sorted list of integers (non-decreasing order).
Output format
An integer representing the number of unique elements after in-place removal of duplicates.
Constraints
- 0 <= len(nums) <= 10^5
- nums is sorted in non-decreasing order
- Must run in O(n) time and O(1) extra space
Samples
Sample 1
Input
remove_duplicates([0, 0, 1, 1, 1, 2, 2, 3, 3, 4])
Output
5
Unique elements are [0,1,2,3,4], so the returned length is 5.